Known are devices for carrying out CPAP (continuous positive airway pressure) therapy. The CPAP therapy is described in Chest. Volume No. 110, pages 1077-1088, October 1996 und in Sleep, Volume No. 19, pages 184-188. A CPAP device applies a positive overpressure of up to approximately 30 mbar into the patient's respiratory airway by means of a compressor, preferably via a humidifier, via a hose and a nose mask. Said overpressure is to ensure that the upper respiratory airway remains fully opened for the whole night, so that no obstructive breathing disorders (apneas) will occur (DE 198 49 571 A1).
FIG. 1 shows a CPAP device 1 and a patient 19. The CPAP device, in turn, comprises a compressor 4, a respiration tube 9, a respiration mask 18, a pressure sensor 11 and a flow sensor 16. For generating an overpressure the compressor contains a turbine 8. The turbine is also designated as a fan, fan unit, compressor, ventilator, or blower. These terms are used as synonyms in this patent. In the illustrated CPAP device, the pressure sensor 11 is positioned in the compressor casing and measures the pressure generated by the turbine. The pressure measuring device may be connected via a flexible tube to the respiration mask, thereby measuring the pressure in the respiration mask. Finally, the pressure sensor may also be located in the respiration mask and connected to the compressor casing via electrical lines. In or near the mask, one or several small holes 2 are provided, resulting in an air flow from the compressor to the holes 2 on time average. This prevents enrichment of CO2 in the respiration tube 9, so that the patient can be supplied with oxygen.
The rotational speed of the turbine 8 is controlled by a microcontroller 5 such that the actual pressure measured with the pressure sensor 11 corresponds to a predetermined desired pressure. The desired pressure is normally preset under the supervision of a physician and is designated as titration pressure. The flow sensor may e.g. be a sensor with a heating wire 17. In another constructional form of the CPAP device, a constriction may be provided in the respiration tube for respiratory flow measurement, the differential pressure being measured via the constriction. The pressure sensors may directly be arranged in the respiration tube or connected to said tube via further pressure measurement tubes. The microcontroller 5 may also take over pressure control.
It was found that the overpressure produced by the CPAP device was considered by patients as an unpleasant resistance against which they had to exhale. That is why control methods were developed for CPAP devices for lowering the desired pressure as much as possible. Such a control is known from WO00/24446. Such a control is based on an algorithm in the case of which at least three pressure values are successively set during an “AutoSet” operation. In cases where the breath volume is independent of the set pressures, the pressures have been too high. When the breath volume increases with the set pressures, the pressures have been too low.
To reduce the overpressure considered to be unpleasant, BiPAP devices and multilevel devices have also been developed. Such a device is described in DE 691 32 030 T2. The pressure is raised through a valve during inhalation and lowered during exhalation. The valve is controlled such that the pressure is kept constant during inhalation and during exhalation. When the valve position is changed only slowly during an inhalation process, this is interpreted as the end of the inhalation process. Inaudible vibrations or pressure changes can be evaluated to determine whether the patient's breathing is regular, irregular or apneic. Moreover, the duration of inhalation and exhalation as well as the flow velocities can be determined. Such information can be stored in a memory. Finally, it is possible to calculate an admittance of respiratory flow divided by pressure. The time-dependent behavior of the admittance can be compared with the stored admittance schemes. The number of the best fitting admittance scheme can be used as an “indicator” for a table which contains the action to be performed, such as an increase in pressure.
WO 94/23780 describes a method for controlling the pressure of a CPAP device. In the absence of breathing disorders during sleep the pressure is reduced gradually. In the presence of sleep disorders, such as apneas, hypopneas or snoring, the pressure is increased. U.S. Pat. No. 5,335,654 and EP 0 934 723 A1 describe a similar method.
EP 0 612 257 B1 also describes an auto CPAP system which detects apneas, hypopneas and unstable breathing to set the pressure.
WO 99/24099 describes a control method for an auto CPAP device which takes into account apneas, hypopneas, reduced respiratory flow and snoring.
According to U.S. Pat. No. 5,740,795 the respiratory flow signal is supplied to a band-limited differentiator. When the output signal of the differentiator exceeds an inhalation threshold or when an exhalation threshold is not reached, an exhalation detection signal or an inhalation detection signal, respectively, is determined.
EP 0 934 723 A1 also refers to the control of a CPAP device on the basis of the detection of apneas and partial occlusion of the upper airway.
Furthermore, DE 101 18 968 describes a control method for CPAP device. DE 101 18 968 is included by reference in this application. The control method first calculates features from a measured respiratory flow curve and a measured actual pressure curve of a CPAP device. Special combinations of the features are combined to form detectors. In the detectors, flags are set when they detect an event, i.e., when they are responsive to the event. The control method will then change the desired pressure on the basis of the event flags of the detectors. The control method includes three different states, namely a normal state, a sensitive state and a leakage state, which can be switched to and fro. Some detectors operate in the sensitive state with parameters differing from the normal state. In the sensitive state, the control method changes when the control method reduces pressure in the normal state. Thanks to the selection of the parameters for the sensitive state the control method will react faster if the actual CPAP pressure is too low. For instance, when the mask is removed, the control will change into the leakage state.
The features comprise the expiration time, a reverse correlation, a mean inspiration volume, a mean curvature of the respiratory flow during inspiration and the frequency of zero passages in the changing portion of the actual CPAP pressure.
During transition from inspiration to expiration a pronounced flank can be detected in the respiratory flow, said flank being used for detecting individual breaths. The local maxima of the first derivative correspond to the maximum increase in the respiratory flow during transition between inspiration and expiration. From the end of the inspiration the beginning of the inspiration is looked for by searching for the first local minimum in the derivative. The expiration time is obtained as the time difference between a minimum of the derivative and the maximum located before the minimum.
For calculating the reverse correlation the most recent breath is compared with preceding breaths by calculating a cross correlation function. The cross correlation function has values between one and minus one, the correlation being equal to one when the two breaths fit each other exactly, and it becomes equal minus one when the curves correlate with one another in a negative way, i.e. when a peak in the respiratory pattern exactly corresponds to a valley in the data piece considered. The mean value over a specific number of local maxima of the cross correlation function before the actual point of time is designated as reverse correlation.
For calculating the mean curvature of the respiratory flow during inspiration the first derivative of the respiratory flow after time is estimated or calculated during inspiration. Subsequently, a straight line is adapted to the first derivative. The slope of this adapted straight line gives the mean curvature of the inspiration.
It has been found that the number of zero passages in the changing portion of the actual CPAP pressure is a reliable feature for snoring, for the pressure regulation of a typical CPAP device is not so fast that it would be capable of correcting snoring noise as well. The zero passages are only counted during the inspiration phase so that the control only reacts in the case of inspiratory snoring. The variance of the actual pressure can also be used for detecting snoring.
According to the teaching of DE 101 18 968 a detector for the cessation of breathing, an apnea detector, a hypopnea detector and a respiratory flow limitation detector are calculated from the features as an indication of an increase in pressure, and a normal detector as an indication of stable breathing and possible pressure reduction. The responses of the detector for the cessation of breathing, of the apnea detector, hypopnea detector and respiratory flow limitation detector are particularly designated as respiratory events in the text below.
The detector for the cessation of breathing will respond if more than two minutes have lapsed without detection of a breath. If this happens more than three times, the automatic pressure regulation will stop.
The apnea detector first determines respirations in which the expiration time is longer than 10 s and which are designated as breathing cessations. The apnea detector will respond if either in two successive cessations of breathing one of the cessations lasts for more than 30 s or if there are more than three successive cessations of breathing. Cessations of breathing are successive if the duration of the intermediate hyperventilation block and breathing period is <60 s.
The non-standardized mean inspiration volume, the reverse correlation and the snoring feature are used for hypopnea detection. The snoring feature, the mean curvature and the reverse correlation are used for detecting respiratory flow limitation. As for details of the hypopnea detector and the respiratory flow limitation detector, reference is made to DE 101 18 968.
The normal detector uses the correlation feature for detecting stable breathing. Stable breathing is present when the desired pressure has not been changed for a predetermined period, e.g. 180 s, and when the reverse correlation is e.g. ≧0.86 during this period.
The fuzzy logic is also known in the prior art. According to conventional logic, logic variables can just assume the states 0 and 1, also designated as “false” and “true”. In fuzzy logic, the fuzzy variables can assume any desired value between 0 and 1, including 0 and 1. The fuzzy logic is above all used in controls which are to take into account the experts' experience.
According to the fuzzy logic, fuzzy variables indicate membership in a set. In a fuzzy control the set corresponds to a specific operative state of the device to be controlled. With the help of the fuzzy logic it is possible to design a control in consideration of a limited number of typical operative states. The fuzzy logic supplies a formalism for the interpolation between the states considered.
It is desirable to provide a method for controlling the pressure of a CPAP device, a CPAP device for carrying out the method, and a storage medium for a corresponding program, which determine a CPAP pressure which is optimum for the patient, on the basis of the time dependence of a patient's respiratory flow curve.